1. Field of the Invention
This invention relates generally to semiconductors, and more particularly to under bump structures having reduced inter-metallic formation.
2. Description of Background
A Controlled Collapse Chip Connection, or C4, is one type of mounting that may be used for mounting semiconductor devices, such as integrated circuits (IC's), Microelectromechanical Systems (MEMS) or other components, to a wafer. C4 utilizes solder bumps instead of wire bonds for the connection. The solder bumps are deposited on chip pads which are located on a top side of a wafer during a final wafer processing step. These solder bumps may be used to connect any type of circuit or device to the wafer.
Environmental legislation in Europe has specifically targeted the wide use of lead in the electronics industry. The directives in Europe require many new electronic circuit boards to be lead free by 1 Jul. 2006, mostly in the consumer goods industry, but in some others as well. With lead-free proliferation driving development activities to focus on lead-free solders, other types of solder are now being utilized.
Many new technical challenges have arisen due to utilizing lead-free solder such as tin. For instance, traditional leaded solders have a significantly higher melting point than lead-free solders, which renders them unsuitable for use with heat-sensitive electronic components and their plastic packaging. To overcome this problem, solder alloys with a high silver content and no lead have been developed with a melting point slightly lower than traditional solders.
Lead-free construction has also extended to components, pins, and connectors. Most of these pins used copper frames, and either lead, tin, gold or other finishes. Tin finishes are the most popular of lead-free finishes.
Having to use lead-free solders has given rise to a problem related to the formation of intermetallics. The formation of intermetallics leads to reliability concerns. The amount of intermetallic increase with time and temperature can result in the degradation of the under bump metallurgy (UBM) interface. Many UBM structures now incorporate nickel (Ni) layers to prevent or reduce intermetallic formation by decreasing the diffusion of copper into the solder bumps. For plated UBM structures, the common practice is to sputter a copper (Cu) seed layer prior to plating. During reflow, solder ball wets expose the edge of the sputtered Cu layer. The intermetallic formation is then dictated by the diffusion path and can result in an effective decrease in UBM pad adhesion. That is, if the sputtered copper layer is not effectively separated from an upper sacrificial layer of the pad (typically formed of Cu), intermetallics may be formed and reduce the strength of the bond to the solder ball placed thereon.